We do live in a world of duality. It can be art and vandalism at the same time. People can make art and commit an act of crime
at the same time.
Admiring graffiti for its artistic merit does not necessarily mean we are condoning the act, or that we wouldn't be upset if someone
did it to our own property.
and from a practical perspective, the railways are typically alot more upset about the trespassing and potential for injury and liability then the extra paint on their freight cars. I would hate to be the conductor or engineer who has to live with killing someone because he was painting in the wrong spot...